Add vertices to existing hook
Keep in mind, that if your hook object is animated, you need to place it in rest/initial position before attaching new vertices (for example switch to frame, where hook is located originaly). Otherwise they could behave incorrectly.

Once did it, enter Edit mode (TAB) and Hook modifier will provide additional options. Select button will show vertices that are already used for hooking. With those selected you should just select new vertices and click Assign. Done.